The congenital long QT syndrome (LQTS) is a hereditary cardiac disorder that causes syncope, seizures, and sudden death from ventricular arrhythmia. At least four ion channel genes have been implicated in LQTS. We have located over 30 families in the Midwest where at least one family member has been clinically identified as having LQTS. Our specific aims are to isolate DNA from patients and immediate family members and: 1. Screen the four ion channel genes to identify mutations which cause LQTS; 2. Prepare selected mutations for expression in systems to identify alterations in channel gating or protein; 3. Identify asymptomatic "carriers" at risk for developing LQTS.